1. Field of the Invention
The invention generally relates to membrane electrode assembly and method for making the same and, particularly, to a carbon nanotube based membrane electrode assembly of a fuel cell and method for making the same.
2. Discussion of Related Art
Fuel cells can generally be classified into alkaline, solid oxide, and proton exchange membrane fuel cells. The proton exchange membrane fuel cell has received more attention and has developed rapidly in recent years. Typically, the proton exchange membrane fuel cell includes a number of separated fuel cell work units. Each work unit includes a fuel cell membrane electrode assembly (MEA), flow field plates (FFP), current collector plates (CCP), as well as related support equipments, such as blowers, valves, and pipelines.
The MEA generally includes a proton exchange membrane and two electrodes separately disposed on two opposite surfaces of the proton exchange membrane. Further, each electrode includes a catalyst layer and a gas diffusion layer. The catalyst layer is configured for being sandwiched between the gas diffusion layer and the proton exchange membrane. The material of the proton exchange membrane is selected from the group consisting of perfluorosulfonic acid, polystyrene sulfonic acid, polystyrene trifluoroacetic acid, phenol formaldehyde resin acid, and hydrocarbons. The catalyst layer includes catalyst materials and carriers. The catalyst materials are selected from the group consisting of metal particles, such as platinum particles, gold particles, and ruthenium particles. The carriers are generally carbon particles, such as graphite, carbon black, carbon fiber or carbon nanotubes. The gas diffusion layer is constituted of treated carbon cloth and carbon paper.
The gas diffusion layer of MEA is mainly formed by a carbon fiber paper. A process of making the carbon fiber paper is by the steps of: mixing carbon fibers, wood pulp, and cellulose fibers; using the mixture to obtain a paper pulp; and then forming the carbon fiber paper from the paper pulp. However, the process of making the carbon fiber paper has the following disadvantages: Firstly, the carbon fibers in the carbon fiber paper are not uniformly dispersed, thereby the gaps therein are uneven resulting in the carbon fibers having a small specific surface area. Thus, the structure restricts the gas diffusion layer to uniformly diffuse the gases, which is needed for the MEA. Secondly, the carbon fiber paper has high electrical resistance, thus, restricting the transfer of electrons between the gas diffusion layer and the external electrical circuit, thereby reducing the reaction activity of the MEA. Thirdly, the carbon fiber paper has poor tensile strength, and is difficult to process.
What is needed, therefore, is a membrane electrode assembly having excellent reaction activity and method for making the same being simple and easy to be applied.